Immersion lithography fluid control system

ABSTRACT

An immersion lithography apparatus includes an optical member, a gap defined between the optical member and a surface disposed opposite the optical member being filled with an immersion liquid, and a fluid control device including a gas outlet through which a gas is supplied to prevent the immersion liquid from entering a surround area external to an exposure area. A flow velocity of the gas supplied from the gas outlet depends on a contact angle between the immersion liquid and the surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Division of U.S. patent application Ser. No. 11/237,650 filedSep. 29, 2005, which is a Continuation of International Application No.PCT/US2004/009911 filed Mar. 29, 2004, which claims the benefit of U.S.Provisional Patent Application No. 60/462,142 filed Apr. 9, 2003. Theentire disclosure of each of the prior applications is herebyincorporated by reference herein in its entirety.

BACKGROUND

This invention relates to an immersion lithography system, such asdescribed in WO99/49504, having a fluid material supplied into the spacebetween a workpiece such as a wafer and the last-stage optical membersuch as a lens of the optical system for projecting the image of areticle onto the workpiece. The supplied fluid material may be purewater and its presence improves the performance of the optical systemand the quality of the exposure.

The fluid material thus supplied into the space between the workpieceand the last-stage optical member tends to rise in temperature due tothe radiation energy from the optical system, thereby causing itscoefficient of refraction to change. If the fluid material remains incontact with the optical member and the workpiece over an extendedperiod of time, furthermore, the fluid material tends to becomepolluted, and this also affects its coefficient of refraction. Also thefluid material tends to leak out of the space between the workpiece andthe last-stage optical member because the workpiece is moved relative tothe last-stage optical member. For these reasons, an immersionlithography system must be provided with an efficient fluid controlsystem for constantly replenishing the lithography fluid.

A problem associated with such a fluid control system for an immersionlithography apparatus is how to control, or contain, the fluid materialwith which the space between the last-stage optical member and theworkpiece is filled.

SUMMARY

A fluid control system according to this invention is for use in animmersion lithography apparatus comprising an optical member, aworkpiece with a surface disposed opposite this optical member with agap between the workpiece and the optical member, a fluid-supplyingdevice for providing a fluid to a specified exposure area in the gap,and what may be broadly referred to as a fluid control device adapted toactivate a force on the fluid supplied into the gap such that the fluidwill be retained within and in the vicinity of the exposure area, andwill be prevented from moving away from the intended limited area, thatis, from entering a specified surrounding area external to the exposurearea.

The force that is to be applied to the fluid has been described as aforce of a kind that can be activated. This means that the force itselfis of a controllable kind and excludes reaction forces from a stationaryobject such as a confining wall. A number of examples of activating aforce on an immersion fluid are considered. One example is to activate agas flow from a pressured gas source such that its hydrodynamic force isarranged to contain the fluid within and in the vicinity of the exposurearea, that is, to prevent the fluid from entering the surrounding areawhere the fluid is not desired.

Another example is to activate a magnetostatic force in which theimmersion fluid is of a magnetically responsive material. Powder of aferromagnetic substance may be added to enhance the magneticcharacteristic of the fluid.

Still another example is to make use of a rheological fluid as theimmersion fluid. In the case of an electrorheological fluid, anelectrostatic field of a suitable intensity may be activated by means ofa suitably positioned pair of capacitor electrodes to increase itsviscosity to practically solidify the fluid. In the case of amagnetorheological fluid, a magnetic field of a suitable intensity maybe activated by means of suitably disposed coils so as to keep theimmersion fluid contained.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the accompanyingdrawings of exemplary embodiments in which like reference numeralsdesignate like elements, and in which:

FIG. 1 is a schematic cross-sectional view of an immersion lithographyapparatus to which methods and systems of this invention may be applied;

FIG. 2 is a flow diagram illustrating an exemplary process by whichsemiconductor devices are fabricated using the apparatus shown in FIG. 1according to the invention;

FIG. 3 is a flowchart of the wafer processing step shown in FIG. 2 inthe case of fabricating semiconductor devices according to theinvention;

FIG. 4 is a schematic vertical view of a portion of an immersionlithography apparatus generally of a structure shown in FIG. 1 includinga fluid control system embodying this invention;

FIG. 5 is a schematic side view of a portion of the immersionlithography apparatus including the fluid control system shown in FIG.4;

FIG. 6 is a schematic side view of a portion of a preferred embodimentof the immersion lithography apparatus including an exhaust manifold;

FIG. 7 is a schematic vertical view of a portion of an immersionlithography apparatus generally of a structure shown in FIG. 1 includinganother fluid control system according to a second embodiment of theinvention;

FIG. 8 is a schematic side view of a portion of the immersionlithography apparatus including a fluid control system according to athird embodiment of the invention; and

FIG. 9 is a schematic side view of a portion of the immersionlithography apparatus including another fluid control system accordingto an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows an immersion lithography apparatus 100 that may incorporatea fluid control system of this invention.

As shown in FIG. 1, the immersion lithography apparatus 100 comprises anilluminator optical unit 1 including a light source such as a KrFexcimer laser unit, an optical integrator (or homogenizer) and a lensand serving to emit pulsed ultraviolet light IL with wavelength 248 nmto be made incident to a pattern on a reticle R. The pattern on thereticle R is projected onto a wafer W coated with a photoresist at aspecified magnification (such as ¼ or ⅕) through a telecentric lightprojection unit PL. The pulsed light IL may alternatively be ArF excimerlaser light with wavelength 193 nm, F₂ laser light with wavelength 157nm or the i-line of a mercury lamp with wavelength 365 nm. In whatfollows, the coordinate system with X-, Y- and Z-axes as shown in FIG. 1is referenced to explain the directions in describing the structure andfunctions of the lithography apparatus 100. For the convenience ofdisclosure and description, the light projection unit PL is illustratedin FIG. 1 only by way of its last-stage optical element (such as a lens)4 disposed opposite to the wafer W and a cylindrical housing 3containing the rest of its components.

The reticle R is supported on a reticle stage RST incorporating amechanism for moving the reticle R in the X-direction, the Y-directionand the rotary direction around the Z-axis. The two-dimensional positionand orientation of the reticle R on the reticle stage RST are detectedby a laser interferometer (not shown) in real time, and the positioningof the reticle R is affected by a main control unit 14 on the basis ofthe detection thus made.

The wafer W is held by a wafer holder (not shown) on a Z-stage 9 forcontrolling the focusing position (along the Z-axis) and the tiltingangle of the wafer W. The Z-stage 9 is affixed to an XY-stage 10 adaptedto move in the XY-plane substantially parallel to the image-formingsurface of the light projection unit PL. The XY-stage 10 is set on abase 11. Thus, the Z-stage 9 serves to match the wafer surface with theimage surface of the light projection unit PL by adjusting the focusingposition (along the Z-axis) and the tilting angle of the wafer W by anauto-focusing and auto-leveling method, and the XY-stage 10 serves toadjust the position of the wafer W in the X-direction and theY-direction.

The two-dimensional position and orientation of the Z-stage 9 (and hencealso of the wafer W) are monitored in real time by another laserinterferometer 13 with reference to a mobile mirror 12 affixed to theZ-stage 9. Control data based on the results of this monitoring aretransmitted from the main control unit 14 to a stage-driving unit 15adapted to control the motions of the Z-stage 9 and the XY-stage 10according to the received control data. At the time of an exposure, theprojection light is made to sequentially move from one to another ofdifferent exposure positions on the wafer W (hereinafter referred to asthe workpiece W) according to the pattern on the reticle R in astep-and-repeat routine or in a step-and-scan routine.

The lithography apparatus 100 described with reference to FIG. 1 is animmersion lithography apparatus and is hence adapted to have a fluid (orthe “immersion liquid”) 7 of a specified kind such as water filling thespace (the “gap”) between the surface of the workpiece W and the lowersurface of the last-stage optical element 4 of the light projection unitPL at least while the pattern image of the reticle R is being projectedonto the workpiece W.

The last-stage optical element 4 of the light projection unit PL may bedetachably affixed to the cylindrical housing 3 and is designed suchthat the liquid 7 will contact only the last-stage optical element 4 andnot the cylindrical housing 3 because the housing 3 typically comprisesa metallic material and is likely to become corroded.

The liquid 7 is supplied from a liquid supply unit 5 that may comprise atank, a pressure pump and a temperature regulator (not individuallyshown) to the space above the workpiece W under a temperature-regulatedcondition and is collected by a liquid recovery unit 6. The temperatureof the liquid 7 is regulated to be approximately the same as thetemperature inside the chamber in which the lithography apparatus 100itself is disposed. Numeral 21 indicates supply nozzles through whichthe liquid 7 is supplied from the supply unit 5. Numeral 23 indicatesrecovery nozzles through which the liquid 7 is collected into therecovery unit 6. However, the structure described above with referenceto FIG. 1 is not intended to limit the scope of the immersionlithography apparatus to which a fluid control system of the inventionis applicable. In other words, a fluid control system of the inventionis applicable to immersion lithography apparatus of many differentkinds. In particular, the numbers and arrangements of the supply andrecovery nozzles 21 and 23 around the light projection unit PL may bedesigned in a variety of ways for establishing a smooth flow and quickrecovery of the immersion liquid 7.

FIGS. 4 and 5 show a fluid control system according to one embodiment ofthe invention as incorporated in an immersion lithography apparatusstructured as shown generally in FIG. 1, characterized as using ahigh-pressure gas for controlling the liquid 7. In FIGS. 4 and 5,numeral 40 indicates the area (hereinafter referred to as the exposurearea) including an illumination field where the light IL from theilluminator optical unit 1 is incident and hence this is the area wherethe liquid 7 should be kept present during the exposure process. Forthis purpose, gas outlets 25 connected to a pressured gas source (notshown) are provided on opposite sides of the area including the exposurearea 40 where the liquid 7 is intended to be confined. In FIG. 5,numeral 45 indicates what may be referred to as the “surrounding area”where the liquid 7 is controlled not to enter. In other words, theliquid 7 may be forced to move with respect to the last-stage opticalelement 4 as the workpiece is scanned but pressured gas from the gasoutlets 25 serves to keep the liquid 7 sufficiently confined such thatit will not move away from the exposure area 40 so much as to reach thespecified surrounding area 45. From the point of view of this invention,therefore, the area specified herein as the surrounding area 45 may beregarded as defining the maximum distance the liquid 7 is permitted tomove away from the exposure area 40.

There is no stringent requirement on the physical arrangement of the gasoutlets 25. The pressured gas may be blown out of individual nozzles, orgrooves may be formed on opposite sides of the exposure area 40 outsidethe supply and recovery nozzles 21 and 23 as shown in FIG. 4 such thatthe pressured gas can be emitted uniformly through one-dimensionallyelongated inlet grooves to form a more uniform pressure wavefront toapply a uniform hydrodynamic force on the liquid 7. In one embodiment,the gas outlets 25 may be provided in the scanning directions asillustrated. In other embodiments, the gas outlets also may be providedin the stepping axis direction (not shown).

In another embodiment, the gas outlets may be provided in the scanningand stepping directions such that the exposure area 40 is surroundedwith the gas outlets. In this case, gas pressure may be differentbetween the gas outlets provided in the scanning directions and the gasoutlets provided in the stepping directions. For example, the gaspressure of the outlets provided in the scanning directions may bestronger while the workpiece W (XY-stage 10) is moved in the scanningdirection, and the gas pressure of the outlets provided in the steppingdirections may be stronger while the workpiece W (XY-stage 10) is movedin the stepping direction. Also, in other embodiments, the gas outletsmay be provided such that the exposure area 40 is encircled with the gasoutlets. In this case, gas pressures may be different on the basis ofposition of the gas outlets, and/or may be changed in accordance withthe motion (such as the moving velocity and the moving direction) of theworkpiece W (XY-stage 10).

In order to minimize the turbulence that may be caused by the gas flowout of the outlets 25, it is desirable to arrange these nozzles or theoutlet grooves 25 diagonally, or obliquely, with respect to the surfaceof the workpiece W, as schematically shown in FIG. 5, although thegas-supplying tubes or pipes (or “supply manifold”) need not be attachedto the rest of the liquid-supplying nozzle system. Generally, the liquidsupply and recovery are designed such that a good balance should exist.If too much liquid is supplied, there will be a leak in the system. Iftoo much recovery is used, it is possible that the gap could be pulleddry or bubbles could be drawn into the gap.

The gas pressure to be supplied depends upon the system configuration.In order to confine the immersion liquid, however, it should have avelocity of approximately 15 to 25 m/sec at the gas/liquid interface. Inone specified embodiment, 20 m/sec was defined. An acceptable range, inview of factors such as the nozzle configuration, may be as wide as2-200 m/sec).

The required flow velocity (gas pressure) also depends on the stagescanning speed, as well as the contact angle between the liquid 7 andthe surface of the workpiece W. The stage scanning speed can vary from10 mm/sec to 1000 mm/sec, or possibly even greater. The contact anglebetween the liquid 7 and the resist material on the workpiece W dependsupon the resist material and also on how it has been treated. A standardArF resist without any top coating will typically have a contact angleof 75°. Adding a topcoat can increase the contact angle to 110° orgreater. With KrF, the contact angle is approximately 60°. For futuretechnology, the contact angle will vary. Generally, the higher thecontact angle, the less pressure is needed, and vice versa. Otherfactors such as the nozzle design and the scanning speed also willaffect the needed pressure.

FIG. 6 shows an embodiment of the invention characterized as having anexhaust manifold 26 for removing the supplied gas in addition to thesupply manifold 25 in order to further control the gas flow which isindicated schematically by way of a dotted arrow. It also has thefeature of reducing the humidity in the scanner chamber by removing thegas that has been directly exposed to the liquid 7.

The gas need not be air. Any similar gas such as nitrogen can be used.Moreover, a gas that absorbs water better than air will be advantageousfrom the standpoint of water containment.

In general, immersion fluid containment is more difficult in thescanning direction as the travel of the wafer stage is greater in thisdirection. An air supply and exhaust manifold can be added to thestepping direction as well, or alternatively just a supply or anexhaust. The invention also can be applied to Twin-Stage-TypeLithography System as is disclosed in U.S. Pat. Nos. 6,262,796 and6,341,007.

FIG. 7 shows a second embodiment of the invention characterized as usinga magnetostatic force to control the liquid 7 by containing it insideand in the immediate vicinity of the exposure area 40 and preventing itfrom reaching the surrounding area 45 as explained above. Water istypically used as the immersion fluid in immersion lithography, andwater is known to be a magnetically responsive liquid, beingdiamagnetic. Thus, a magnetic force can be applied on such a fluidmaterial by providing a suitable magnetic field over the area where theliquid 7 is confined. FIG. 7 shows an example in which a plurality ofelectromagnetic coils 47, serving together as a magnetic fieldgenerator, are arranged around the exposure area 40 and a magnetic fieldis generated so as to control the flow of the liquid 7. For theconvenience of disclosure, the circuit for passing currents throughthese coils 47 is omitted.

In order to enhance the magnetically responsive characteristic of theimmersion fluid such as water, powder of a ferromagnetic substance suchas Ni, Fe and Co may be added to the liquid 7 to the extent that it willnot adversely affect the transparency and other optical characteristicsof the liquid 7.

The invention according to a third embodiment is characterized as usinga rheological fluid, such as an electrorheological fluid (ERF) or amagnetorheological fluid (MRF) between the last-stage optical element 4and the workpiece W, as the immersion fluid. An ERF is characterized ashaving the property of very low viscosity (i.e., less than 10 Pa-s)under normal conditions but very high viscosity when subjected to anelectric field. An MRF is characterized as having the property ofsimilarly very low viscosity under normal conditions but very highviscosity when subjected to a magnetic field. In the above, theexpression “very high viscosity” means that these fluids become aso-called Bingham solid with viscosity no longer measurable.

FIG. 8 shows a fluid control system according to the third embodiment ofthe invention for immersion lithography characterized as using an ERF 70and having capacitor electrodes 50 as an example of what is hereinsometimes broadly referred to as a field generator that, in thisinstance, is a generator of an electrostatic field. The capacitorelectrodes 50 are disposed as shown in FIG. 8 and connected to a voltagesource 52 so as to generate an electrostatic field of 3-4 kV/mm which isconsidered sufficiently strong for solidifying the kind of ERF commonlyavailable currently in a region surrounding the exposure area 40 suchthat the ERF 70 will remain in the liquid phase in the exposure area 40but will solidify as indicated by numeral 71 in the surrounding areasuch that the ERF 70 in the liquid phase is contained within a regioncentering around the exposure area 40 and is prevented from entering thesurround area.

FIG. 9 shows another fluid control system according to the thirdembodiment of the invention for immersion lithography characterized asusing an MRF 75 and having a magnetic field generator such as a coil 60for generating a magnetostatic field of about 0.1-0.8 Tesla over thesurface of the workpiece W and another field generator (herein referredto as the opposite field generator) 62 disposed as shown in FIG. 9 so asto generate a magnetic field equal to but oriented opposite to themagnetic field generated by the coil 60 within and about the exposurearea 40 such that when both these coils 60 and 62 are switched on, themagnetic fields generated thereby effectively cancel each other withinand in the vicinity of the exposure area 40. As a result, the portion ofthe MRF 75 within and in the vicinity of the exposure area 40 remains inthe liquid phase but the MFR 75 is solidified, as indicated by numeral76 in the surrounding area due to the magnetic field generated by thecoil 60 such that the MRF 75 in the liquid phase is contained within aregion centering around the exposure area 40 and is prevented fromentering the surrounding area.

As the workpiece W is scanned under the light projection unit PL, thelocation of the opposite canceling field, which is fixed to the lightprojection unit PL, moves along the surface of the workpiece W. Theopposite field provided by the opposite field generator 62 serves todesolidify and resolidify the fluid on the surface of the workpiece Wsuch that the fluid 75 remains in the liquid phase within and in thevicinity of the exposure area 40.

Although the invention has been described above with reference to alimited number of embodiments, these embodiments and illustratedexamples are not intended to limit the scope of the invention. Manymodifications and variations are possible. For example, theelectromagnets 47 in FIG. 7 need not be arranged as illustrated.Depending on the kind of immersion fluid and its flow speed, anaccordingly more suitable arrangement may be selected by a personskilled in the art.

FIG. 2 is referenced next to describe a process for fabricating asemiconductor device by using an immersion lithography apparatusincorporating a fluid control system embodying this invention. In step301 the device's function and performance characteristics are designed.Next, in step 302, a mask (reticle) having a pattern is designedaccording to the previous designing step, and in a parallel step 303, awafer is made from a silicon material. The mask pattern designed in step302 is exposed onto the wafer from step 303 in step 304 by aphotolithography system such as the systems described above. In step 305the semiconductor device is assembled (including the dicing process,bonding process and packaging process), then finally the device isinspected in step 306.

FIG. 3 illustrates a detailed flowchart example of the above-mentionedstep 304 in the case of fabricating semiconductor devices. In step 311(oxidation step), the wafer surface is oxidized. In step 312 (CVD step),an insulation film is formed on the wafer surface. In step 313(electrode formation step), electrodes are formed on the wafer by vapordeposition. In step 314 (ion implantation step), ions are implanted inthe wafer. The aforementioned steps 311-314 form the preprocessing stepsfor wafers during wafer processing, and selection is made at each stepaccording to processing requirements.

At each stage of wafer processing, when the above-mentionedpreprocessing steps have been completed, the following post-processingsteps are implemented. During post-processing, initially, in step 315(photoresist formation step), photoresist is applied to a wafer. Next,in step 316 (exposure step), the above-mentioned exposure device is usedto transfer the circuit pattern of a mask (reticle) onto a wafer. Then,in step 317 (developing step), the exposed wafer is developed, and instep 318 (etching step), parts other than residual photoresist (exposedmaterial surface) are removed by etching. In step 319 (photoresistremoval step), unnecessary photoresist remaining after etching isremoved. Multiple circuit patterns are formed by repetition of thesepreprocessing and post-processing steps.

While a lithography system of this invention has been described in termsof several preferred embodiments, alterations, permutations, and varioussubstitute equivalents are possible. There are many alternative ways ofimplementing the methods and apparatus of the invention.

1. An immersion lithography apparatus comprising: an optical member, agap defined between the optical member and a surface disposed oppositethe optical member being filled with an immersion liquid; and a fluidcontrol device including a gas outlet through which a gas is supplied toprevent the immersion liquid from entering a surround area external toan exposure area, wherein a flow velocity of the gas supplied from thegas outlet depends on a contact angle between the immersion liquid andthe surface.
 2. The immersion lithography apparatus according to claim1, wherein the gas supplied from the gas outlet is removed.
 3. Theimmersion lithography apparatus according to claim 2, further comprisingan exhaust manifold which removes the supplied gas.
 4. The immersionlithography apparatus according to claim 1, wherein the gas includesair.
 5. The immersion lithography apparatus according to claim 1,wherein the gas includes nitrogen.
 6. The immersion lithographyapparatus according to claim 1, further comprising a liquid recoveryunit which recovers the supplied immersion liquid, wherein the immersionliquid is recovered inside the gas outlet.
 7. The immersion lithographyapparatus according to claim 1, wherein a flow velocity of the gassupplied from the gas outlet is 2 to 200 m/sec at a gas/liquidinterface.
 8. A method for patterning a wafer, the method comprising:exposing the wafer utilizing the immersion lithography apparatus ofclaim 1 by projecting a pattern image onto the wafer through the opticalmember.
 9. An immersion lithography apparatus comprising: an opticalmember, a gap defined between the optical member and a surface disposedopposite the optical member being filled with an immersion liquid; and afluid control device including gas outlets through which a gas issupplied to prevent the immersion liquid from entering a surround areaexternal to an exposure area, wherein gas pressures are different on thebasis of positions of the gas outlets.
 10. The immersion lithographyapparatus according to claim 9, wherein the gas supplied from the gasoutlets is removed.
 11. The immersion lithography apparatus according toclaim 10, further comprising an exhaust manifold which removes thesupplied gas.
 12. The immersion lithography apparatus according to claim9, wherein the gas includes air.
 13. The immersion lithography apparatusaccording to claim 9, wherein the gas includes nitrogen.
 14. Theimmersion lithography apparatus according to claim 9, wherein the gasoutlets are provided such that the exposure area is surrounded with thegas outlets.
 15. The immersion lithography apparatus according to claim14, further comprising a liquid recovery unit which recovers thesupplied immersion liquid, wherein the immersion liquid is recoveredinside the gas outlets.
 16. A method for patterning a wafer, the methodcomprising: exposing the wafer utilizing the immersion lithographyapparatus of claim 9, by projecting a pattern image onto the waferthrough the optical member.
 17. An immersion lithography apparatuscomprising: an optical member, a gap defined between the optical memberand a surface disposed opposite the optical member being filled with animmersion liquid; and a fluid control device including gas outletsthrough which a gas is supplied to prevent the immersion liquid fromentering a surround area external to an exposure area, wherein gaspressures are changed in accordance with motion of the surface.
 18. Theimmersion lithography apparatus according to claim 17, wherein themotion of the surface includes a moving velocity.
 19. The immersionlithography apparatus according to claim 17, wherein the motion of thesurface includes a moving direction.
 20. The immersion lithographyapparatus according to claim 17, wherein the gas supplied from the gasoutlets is removed.
 21. The immersion lithography apparatus according toclaim 20, further comprising an exhaust manifold which removes thesupplied gas.
 22. The immersion lithography apparatus according to claim17, wherein the gas includes air.
 23. The immersion lithographyapparatus according to claim 17, wherein the gas includes nitrogen. 24.The immersion lithography apparatus according to claim 17, wherein thegas outlets are provided such that the exposure area is surrounded withthe gas outlets.
 25. The immersion lithography apparatus according toclaim 24, further comprising a liquid recovery unit which recovers thesupplied immersion liquid, wherein the immersion liquid is recoveredinside the gas outlets.
 26. A method for patterning a wafer, the methodcomprising: exposing the wafer utilizing the immersion lithographyapparatus of claim 17 by projecting a pattern image onto the waferthrough the optical member.